1. Field of the Invention
This invention relates to a method of stabilizing the emitted electron beam current from the cathode of a field emission electron gun over a long period of time.
2. Description of the Prior Art
It is well known that electrons are field emitted from one surface of metallic or non-metallic materials when a strong electric field in excess of about 10.sup.7 volt/cm is applied to the surface. Although cathodes utilizing such a field emission effect are characteristic of luminances about 10.sup.3 times as high as cathodes employing the thermionic emission effect, the former cathodes have some difficult problems.
One of the problems is that the radius of curvature of the cathode tip is generally required to be made very small, for example, 1,000 to 3,000 A in order to facilitate application of such a strong electric field as mentioned above, and therefore field emission cathode materials are required which have a sufficiently high mechanical strength. This problem has been solved by using tungsten for the cathode material.
The most serious one of the problems encountered in electron guns employing a field emission cathode is a problem on how stable electron emission from the cathode can be sustained for a long time. Generally, in field emission electron guns a stable electron beam can be obtained only for a very short time since the density of the emitted electron beam tends to be changed in dependence on the conditions of the surface of the cathode. One of the causes of this is considered to be the absorption of residual gas in a vacuum atmosphere onto the cathode surface. Another cause is considered to result from the residual gas being ionized by the emitted electron beam and positive ions thus produced impinging on the cathode surface to cause turbulence of the surface conditions. Therefore, stabilization of electron emission from field emission cathodes will require an ambient atmosphere having a low pressure, generally a very high vacuum in excess of 10.sup.-.sup.11 Torr. However, in the case of using a field emission cathode in physics and chemistry instruments such as an electron microscope, it is very difficult to obtain such a high vacuum because the electron gun parts and various electro-optic system parts are included in the same evacuated vessel, and thus gas is emitted from these parts to the vacuum atmosphere. For this reason, it is normal practice to conduct field emission in an atmosphere of about 10.sup.-.sup.10 Torr. However, field emission in such a vacuum atmosphere has the disadvantage that the resultant field emitted electron beam current is not sufficiently stable, i.e., it shows fluctuations of a few percent.